1. CSCE 351: Operating System Kernels
Process
CSCE 351: Operating System Kernels

2. Major Requirements of an OS
Interleave the execution of several processes to maximize processor utilization while providing reasonable response time
Allocate resources to processes
Support interprocess communication and user creation of processes

3. Processes The Process Model
Multiprogramming of four programs
Conceptual model of 4 independent, sequential processes
Only one program active at any instant

5. Two-State Process Model
Process may be in one of two states
Running
Not-running

6. Two-State Process Model (2)
Not-running
ready to execute
Blocked
waiting for I/O
Dispatcher cannot just select the process that has been in the queue the longest because it may be blocked

7. Not-Running Process in a Queue

8. Process Creation Principal events that cause process creation
System initialization
Execution of a process creation system
User request to create a new process
Initiation of a batch job

9. Process Termination Conditions which terminate processes
Normal exit (voluntary)
Error exit (voluntary)
Fatal error (involuntary)
Killed by another process (involuntary)

10. Process Hierarchies
Parent creates a child process, child processes can create its own process
Forms a hierarchy
UNIX calls this a "process group"
Windows has no concept of process hierarchy
all processes are created equal

11. A Three-State Model (1) Possible process states
running
blocked
ready
Transitions between states shown

12. A Three-State Model (2) Lowest layer of process-structured OS
handles interrupts, scheduling
Above that layer are sequential processes

13. A Five-State Model
Running
Ready
Blocked
New
Exit

14. A Five-State Model

15. Using Two Queues

16. Using Multiple Queues

17. Suspended Processes
Processor is faster than I/O so all processes could be waiting for I/O
Swap these processes to disk to free up more memory
Blocked state becomes suspend state when swapped to disk
Two new states
Blocked, suspend
Ready, suspend

18. One Suspend State

19. Two Suspend States

20. Reasons for Process Suspension

22. Operating System Control Structures
Information about the current status of each process and resource
Tables are constructed for each entity the operating system manages

23. Memory Tables Allocation of main memory to processes
Allocation of secondary memory to processes
Protection attributes for access to shared memory regions
Information needed to manage virtual memory

24. I/O Tables I/O device is available or assigned Status of I/O operation
Location in main memory being used as the source or destination of the I/O transfer

25. File Tables Existence of files Location on secondary memory
Current Status
Attributes
Sometimes this information is maintained by a file-management system

26. Process Table Where process is located
Attributes necessary for its management
Process ID
Process state
Location in memory

27. Fields of a process table entry

28. Process Location Process includes set of programs to be executed
Data locations for local and global variables
Any defined constants
Stack
Process control block
Collection of attributes
Process image
Collection of program, data, stack, and attributes

30. Process Control Block (1)
Process identification
Identifiers
Numeric identifiers that may be stored with the process control block include
Identifier of this process
Identifier of the process that created this process (parent process)
User identifier

31. Process Control Block (2)
Processor State Information
User-Visible Registers
A user-visible register is one that may be referenced by means of the machine language that the processor executes. Typically, there are from 8 to 32 of these registers, although some RISC implementations have over 100.

32. Process Control Block (3)
Processor State Information
Control and Status Registers
These are a variety of processor registers that are employed to control the operation of the processor. These include
•Program counter: Contains the address of the next instruction to be fetched
•Condition codes: Result of the most recent arithmetic or logical operation (e.g., sign, zero, carry, equal, overflow)
•Status information: Includes interrupt enabled/disabled flags, execution mode

33. Process Control Block (4)
Processor State Information
Stack Pointers
Each process has one or more last-in-first-out (LIFO) system stacks associated with it. A stack is used to store parameters and calling addresses for procedure and system calls. The stack pointer points to the top of the stack.

34. Process Control Block (5)
Process Control Information
Scheduling and State
Process state: defines the readiness of the process to be scheduled for execution (e.g., running, ready, waiting, halted).
Priority: One or more fields may be used to describe the scheduling priority of the process. In some systems, several values are required (e.g., default, current, highest-allowable)
Scheduling-related information: This will depend on the scheduling algorithm used. Examples are the amount of time that the process has been waiting and the amount of time that the process executed the last time it was running.
•Event: Identity of event the process is awaiting before it can be resumed

35. Process Control Block (6)
Process Control Information
Data Structuring
A process may be linked to other process in a queue, ring, or some other structure. For example, all processes in a waiting state for a particular priority level may be linked in a queue. A process may exhibit a parent-child (creator-created) relationship with another process. The process control block may contain pointers to other processes to support these structures.

36. Process Control Block (7)
Process Control Information
Interprocess Communication
Various flags, signals, and messages may be associated with communication between two independent processes. Some or all of this information may be maintained in the process control block.
Process Privileges
Processes are granted privileges in terms of the memory that may be accessed and the types of instructions that may be executed. In addition, privileges may apply to the use of system utilities and services.

37. Process Control Block (8)
Process Control Information
Memory Management
This section may include pointers to segment and/or page tables that describe the virtual memory assigned to this process.
Resource Ownership and Utilization
Resources controlled by the process may be indicated, such as opened files. A history of utilization of the processor or other resources may also be included; this information may be needed by the scheduler.

38. Process Control Block (9)

39. Processor State Information
Contents of processor registers
User-visible registers
Control and status registers
Stack pointers
Program status word (PSW)
contains status information
Example: the EFLAGS register on Pentium machines

40. Pentium II EFLAGS Register

41. Modes of Execution User mode System mode, control mode, or kernel mode
Less-privileged mode
User programs typically execute in this mode
System mode, control mode, or kernel mode
More-privileged mode
Kernel of the operating system

42. Process Creation Assign a unique process identifier
Allocate space for the process
Initialize process control block
Set up appropriate linkages
Ex: add new process to linked list used for scheduling queue
Create of expand other data structures
Ex: maintain an accounting file

43. When to Switch a Process
Clock interrupt
process has executed for the maximum allowable time slice
I/O interrupt
Memory fault
memory address is in virtual memory so it must be brought into main memory

44. When to Switch a Process
Trap
error occurred
may cause process to be moved to Exit state
Supervisor call
such as file open

45. Change of Process State (1)
Save context of processor including program counter and other registers
Update the process control block of the process that is currently running
Move process control block to appropriate queue - ready, blocked
Select another process for execution

46. Change of Process State (2)
Update the process control block of the process selected
Update memory-management data structures
Restore context of the selected process

47. Change of Process State (3)
Skeleton of what lowest level of OS does when an interrupt occurs

48. Execution of the Operating System
Non-process Kernel
Execute kernel outside of any process
Operating system code is executed as a separate entity that operates in privileged mode
Execution Within User Processes
Operating system software within context of a user process
Process executes in privileged mode when executing operating system code

50. Execution of the Operating System
Process-Based Operating System
Major kernel functions are separate processes
Useful in multi-processor or multi-computer environment

51. UNIX SVR4 Process Management
Most of the operating system executes within the environment of a user process

52. UNIX Process States

54. Summary In this presentation we discussed
Process life-cycle
Process swapping mechanism
Basic data structure that maintain process
At this point, we just finished chapter 3 in the book